The New Radio (NR) definition in 3GPP (3rd Generation Partnership Project) for mobile communication systems will encompass a variety of deployment scenarios envisioned for 5G (fifth generation) mobile communication systems. MIMO (multiple input, multiple output) communication systems can be used for 5G TDD (time division duplex) air interfaces. In particular, massive MIMO reciprocity-based TDD air interfaces allow for symbol-level switching and potential configurability that in turn allow for features to support various aspects of 5G air interfaces, for example, enhanced Mobile BroadBand (eMBB), massive Machine Type Communications (mMTC) and Ultra-Reliable and Low Latency Communications (URLLC). Example embodiments for massive MIMO communication systems and related reciprocity-based TDD air interfaces are described within U.S. Published Patent Application 2015/0326286, entitled “MASSIVE MIMO ARCHITECTURE,” U.S. Published Patent Application 2015/0326383, entitled “SYNCHRONIZATION OF LARGE ANTENNA COUNT SYSTEMS,” and U.S. Published Patent Application 2015/0326291, entitled “SIGNALING AND FRAME STRUCTURE FOR MASSIVE MIMO CELLULAR TELECOMMUNICATION SYSTEMS,” each of which is hereby incorporated by reference in its entirety.
Many-element antenna arrays, in the order of hundreds of elements per array, are a core component of 5G communications systems. The massive MIMO systems utilize time division duplex (TDD) to separate uplink and downlink. A key implementation requirement is to use knowledge about the wireless channel, obtained in the uplink, to precode signals for the downlink transmission. This technique is referred to as “reciprocity-based precoding.” Precoding ensures that multiple mobile stations, with only a few antennas each, receive downlink signals without multi-user interference.
Typically, different radio components are used for uplink and downlink transmission at the base station. Their impact needs to be calibrated accurately (reciprocity calibration) such that uplink channel information can be used for downlink precoding. Inaccurate calibration results in multi-user interference in the downlink, which results in reduced data rates.
Calibration can be implemented internally within the base station array by measuring the channel between individual base station array antenna elements. These measurements need to be accurate. Antenna array calibration accuracy can be severely reduced if the link between a certain antenna element and a reference antenna element is attenuated, for instance, due to array geometry.